A polyarylene sulfide (hereinafter sometimes referred to as “PAS”), especially polyphenylene sulfide is known as an engineering plastic which is excellent in mechanical strength, heat resistance, flame retardancy, solvent resistance and the like and which has good electrical characteristics and high rigidity. Thus it is widely employed as a variety of materials such as automobiles parts, electronic parts, electrical parts and mechanical parts.
Such PAS resin as stated above has hitherto been produced by a batch process, but there has recently been an increasing demand for continuous polymerization process for the sake of improvement in the production efficiency. There are disclosed as a continuous polymerization process, for instance, U.S. Pat. Nos. 4,056,515, 4,060,520, 4,066,632 and the like. However, any of the above-disclosed process involves the problem in that the molecular weight of the PAS thus obtained is markedly low.
On the other hand, there is proposed for the purpose of increasing its molecular weight, a continuous polymerization process wherein a phase separation agent (water, sodium acetate, an alkali metal salt or the like) is used to separate into a polymer phase and a solvent phase (refer to Japanese Patent Application Laid-Open No. 169844/1997 (Heisei 9).
In the case of continuous polymerization process wherein such a phase separation agent is used, there is such a problem that polymerization reaction liquid becomes a phase separation state in which a polymer phase and a solvent phase are separated from each other, the polymer phase settles owing to difference in specific gravity in a place difficult to be influenced by shearing force due to stirring and the like such as the bottom of a polymerization vessel and inside of piping, thereby for instance, making it impossible to keep the compositional ratio (concentration) of polymer phase/solvent phase at a constant level during the transfer of the polymerization reaction liquid, and as a result variation in the polymer concentration in the polymerization vessel leads to variation in the the molecular weight, thus making it difficult to produce PAS which is stabilized in its molecular weight.
With regard to the above-mentioned problem, the process in the foregoing Japanese Patent Application Laid-Open No. 169844/1997 (Heisei 9) in which a proposal is made of a method for separately withdrawing the polymer phase and solvent phase fails to sufficiently cope with the above-mentioned problem because of the use of a special shape of a reactor, a complicated piping structure, difficulty in flow rate control and the like factors. In such circumstances, it has been eagerly desired to develop a continuous polymerization process which uses a phase separation agent and which is capable of discharging the polymer phase and solvent phase from a polymerization vessel at a constant ratio and as a result, maintaining the PAS composition (concentration) in the polymerization vessel at a constant level at all times.